You can use this js code. Just change .post-thumb img with your img.
$('.post-thumb img').each(function(){ // Note: {.post-thumb img} is css selector of the image tag
var t = $(this),
s = 'url(' + t.attr('src') + ')',
p = t.parent(),
d = $('<div></div>');
t.hide();
p.append(d);
d.css({
'height' : 260, // Note: You can change it for your needs
'background-size' : 'cover',
'background-repeat' : 'no-repeat',
'background-position' : 'center',
'background-image' : s
});
});